1. Field of the Invention:
The present invention relates to techniques for reducing the weight of and intensifying the strength of a crankshaft which is mounted in a V-6 type engine for a small-, intermediate- or large-sized automobile.
2. Description of the Prior Art:
As is well known, automobiles are provided with various types of engine. A small-, intermediate- or large-sized car is provided with, for example, a so-called V-6 type engine in which six cylinders are arranged such that adjacent cylinders form a V shape at an angle of 60.degree.. The crankshaft of this type of engine is similar to that of FIG. 1, but includes additional counterweights 63, 64, 66, and 67 coupled to, respectively, arms 33, 34, 36 and 37, as indicated in FIG. 2.
More specifically, the crankshaft 1 is provided with first to fourth journals 21, 22, 23, 24, from the left-hand side to the right-hand side of FIG. 1, which correspond to respective cylinder-block bearings. Pins 41, 42, 43, 44, 45, 46 corresponding to respective connecting rods are integrally connected to the crankshaft 1 between the respective adjacent journals through first to ninth arms 31, 32, 33, 34, 35, 36, 37, 38, 39.
It is to be noted that the rear portion of the fourth journal 24, which is at the terminating end of the crankshaft 1, is integrally formed with a mounting surface 6 for mounting a flywheel 5.
The so-called nine-balance arrangement as viewed in the lateral direction of the crankshaft 1 in which nine counterweights are employed may schematically be shown as in FIG. 2, while the schematic arrangement as viewed in the axial direction of the crankshaft 1 may be as shown in FIG. 3.
The mass of each of the counterweights which are provided as shown in FIG. 2 in order to balance the inertia couple produced by the rotating and reciprocating mass is represented by the vector of inertia force in FIG. 3. More specifically, the first and ninth counterweights 61, 69 whose vectors are opposite in direction to each other are equivalently balanced by each other. The second, fifth and eighth counterweights 62, 65, 68 are equivalently arranged such as to cross each other at 120.degree. and therefore are similarly balanced by each other. Moreover, the third and sixth counterweights 63, 66 and the fourth and seventh counterweights 64, 67 are equivalently arranged in axial symmetry with each other with respect to the arm 35 such that the third and seventh counterweights 63, 67 cross each other at 60.degree. and the fourth and sixth counterweights 64, 66 cross each other at 60.degree.; therefore, the pairs of counterweights 63, 66 and 64, 67 are similarly balanced by each other. Thus, the balance between the corresponding counterweights is reliably kept.
As described above, the conventional crankshaft structure is advantageous in that the balance between the corresponding counterweights is ideally kept and that the crankshaft structure can well afford to bear large surface pressure and also to resist, a large bending and shearing stress. The conventional crankshaft structure, however, has an extremely large weight owing to the fact that all the arms are provided with counterweights so as to be fully balanced. For this reason, the natural frequency of torsional vibration of the crankshaft cannot adequately be increased, resulting disadvantageously in an increase in torsional vibration in the high speed region (engine). Moreover, the fact that the crankshaft structure has a large weight is unfavorable in terms of responsiveness to acceleration and disadvantageously deteriorates the fuel consumption rate.